Porcelain enamel



Patented July 29, 1941 I w" PORCELAIN ENAMEL Monroe J. Bahnsen, Lakewood, and Eugene E.

Bryant, Bedford, Ohio, assignors to Ferro 'Enamel Corporation, Cleveland, Ohio, a corporation of Ohio No Drawing. Application December 28, 1939,

Serial No. 311,362

' 2 Claims.

The present invention relates as indicated to porcelain enamels and particularly enamels having high covering power and characterized further in that they are free or substantially free of antimony.

Opaque enamels, i. e., those which have satisv factory covering power and give high reflectance, as measured by present-day commercial standards, generally contain substantial amounts of antimony. Antimony-bearing compounds and antimony complexes are generally relied upon to impart opacity to enamels for the reason that these substances are white and of a different refractive index than the glass in which they are uniformly distributed. To obtain a high degree of opacity in antimony-bearing enamels, it is necessary to smelt the raw batch at as low a tem-. perature as possible and still melt the glass to a viscous liquid condition. These enamels thus made under such conditions must be very carefully smelted in order to' insure uniform distribution of the antimony-bearing materials throughout the glass'body.

The low smelting temperatures used on antimony-bearing glasses are necessary because at higher temperatures there is-a tendency for some of the antimony-bearing material to be volatilized and thereby lost and also the remaining antimony-bearing materials dissolved to some extent in the glass matrix and thus losetheir opacifying effect.

As the amount of antimony added in the. raw batch is increased, in order to increase opacity, the amount of glass-forming materials or fluxes is, of course, proportionately decreased. This also tends to increase the viscosity of the antimony-bearing glasses during smelting.

Commercial practice today utilizes about the maximum possible amount of antimony-bearing materials in the smelter. the upper limit, as indicated, being determined by the 'difllculties en:

of the past, and can be used today only by very careful control in their manufacture and use.

A further difliculty with low firing, high viscosity enamels is their tendency to blister during firing. It has been quite definitely shown that high viscosity and undersmelting are primary causes of blistering.

It is a principle of this invention, therefore, to provide an enamel characterized by high opacity. Due tothe omission of antimony from'enamels comprising this invention, they are characterized by high fluidity and are, therefore, more like the clearglasses originally employed as enamels .with, however, the added property of high opacity.

The omission of antimony from the enamel also very substantially reduces its cost.

Other objects of this invention will appear as the description proceeds. I

To the accomplishment of the foregoing and related ends, said invention, then, consists of the "steps hereinafter fully described and particularly pointed out in the claims.

The following description sets forth in detail one approved method of carrying out the invention, such disclosed method, however, constituting but one of various ways in which the principle of the invention may be used.

Broadly stated, this invention comprises the discovery that bythe use of substantial amounts of phosphorous-containing compounds in antimony-free frits and particularly in frits of a relatively narrow range of composition, the frit which, as normally smelted, is a substantially clear glaze, but upon firing on the ware. has a very substantial degree of opacity. These improved frits, when combined with various mill additions, are capable. of producing extremely high opacity heretofore not possible excepting with difliculty and much more expensive materials.

A frit made in accordance with the present in- I vention may be produced from a raw batch mixincreased in accordance with conventional methture having the following range of compositions:

The calculated oxide compositions of the resultant frit produced from raw batch compositions within the above range is as follows:

The following table gives five specific examples of raw batch mixes coming within the range of Table A above:

Table C Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5

Silica sand 2. 85 None None None None Dehydrated borer 18.15 17.46 16.00 15.4 15.0 24. 9) 32.50 32.00 32.4 32.8 19.30 20.70 20.4 20.6 20.9

1.15 1.22 1.2 1.2 1.25 4. 55 4. 88 4. 8 T. 3 5.00 15.25 11.38 13.7 11. 5 13.10 5.30 5. 69 5.6 5.7 5.00 4.90 3.25 3.2 3.2 3.25 Zinc oxide 2.65 2.84 2.8 2.8 2.90

The calculated oxide content of the frit smelted from the raw batch mixtures given in Table C above is approximately as follows:

Table D Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5

31. 64 32. as 33. 2o 32. 86 34 30 12.83 13.80 13.65 13.80 14.) i2. 65 12. 08 i1. 05 10. 65 10. 150 12.18 10. 58 10. 87 9. 94 10.56 2. 50 3. 25 3. 2) 3. 24 3. 28 6.14 6. 59 6. 49 8. 33 5. 30 7. 67 7. 96 7. 85 7. 81 8. 00 2.05 2.84 2.80 2.80 2.90. 12.07 9. 28 10.64 10. 53 10. 39 2. 43 2. 62 2. 56 2. 61 2. 20

The foregoing tables of compositions give specific examples which have been eminently successful commercially. It will be evident to those familiar with the art that certain changes may always be made in the proportionate amounts of the specific components above-mentioned, as, for example, the feldspar content may be increased, in which case, there would be corresponding reductions in the amount of silica and alkalis used and similarly, sodium fiuo silicate may be varied v with corresponding variations in the amount of borax used and also cryolite may be used in complete or partial substitution for the sodium fiuo silicate. In the appended claims, therefore, where reference is had to specific compositions, it will be understood that alterations therein, such as the above specified, are to be included, since they do not materially affect the results achieved.

The raw mix constituents within the proportions given in the foregoing tables are smelted in a conventional manner, as for example, in a smelter oi' the type disclosed in U. 8. Patent Nos. 2,097,378 and 2,097,379. These enamels are usually smelted at a temperature of approximately 2250 F. and the rate of cooling, after smelting, determines the character of the frit insofar as its opacity is concerned, although it does not materially affect the opacity of the enamel on the ultimate work-piece. For example, if the smelted frit is very rapidly cooled, as by discharge of a stream of the same into the water bath, a substantially clear frit is produced. If, however, the enamel is permitted to cool rather slowly after smelting, and permitted to anneal" at a temperature on the order of about 1400 F. to 1800 F., the resultant frit will be quite white, indicating the nature of the action which takes place when the frit, without the addition of any opacifier, is fired onto a metallic article. It is believed that the slow cooling develops a crystalline structure highly refractive in its nature, thus accounting for the opacity developed by slow cooling. While we do not wish to be limited to any particular theory on which the results of the present invention are achieved, nevertheless, the action of the glass in the smelter during the annealing stage, above referred to, seems to confirm this conclusion.

As indicated, a frit produced in accordance with this invention which, if quickly cooled from a high temperature, is a clear frit or alternatively, if fritted from an annealing temperature is a white frit, will, when deposited in a conventional manner and fired on the metallic article, produce a white enamel of unexpected covering power. Thus, for example, when fired on a sheet metal article at a temperature of 1500 F. to 1520 F., 64 grams per square foot will have a total reflectance of 79%. These results are based on two coats of application over conventional blue ground coat. The above figures give an indication of the remarkable covering power of this enamel. even without the use of any of the many available mill addition opacifiers with which this enamel may be employed. Since the employment of a small amount of opacifier as a mill addition is desirable from an economic standpoint, it is within the contemplation of our invention to use the enamel of thepresent invention in conjunction with mill additions of various types.

The mill additions used will include the usual electrolytes orsetting-up salts, such as magnesium carbonate, potassium carbonate, sodium nitrite and sodium aluminate usually employed in amounts up to /2 lb. per lbs. of frit.

Clay is used as a mill addition for the purposes for which the clay is used as a mill addition with enamels generally. A small amount of bentonite in addition or in partial substitution of the clay will be found helpful in improving the setting-up properties of the enamel and increasing the strength of the dry bisque.

The compositions of various zirconium opacifiers will be found in the following U. S. patents: 1,562,890, 1,789,311, 2,083,024, 2,100,337, 2,102,627. Opacifiers of this character i. e. zirconium-bearing opacifiers will generally be found most useful in amounts up to about 5%, since concentrations higher than 5% are generally not as economical as the lower concentrations.

A typical example of a mill addition is as follows:

Per 100 lbs. of frit fired at 1500 F. to 1520 F. will produce an ulti- As previously indicated, minor amounts of" bentonite, on the order of 2 ozs. per hundred pounds of frit may be advantageously employed on occasion. The frit and the mill additions are ground to a fineness such that there will be a 2-4 gram residue on a 200 mesh screen from a 50 cc. sample at a specific gravity of 1.82.

The gum tragacanth in the amounts specified above as a mill addition is useful for the purpose of giving film strength in the dry bisque.

By the foregoing specific examples of mill additions and particularly opacifiers which may be employed in conjunction with the enamel of our invention, we, of course, do not intend to be limited to the use of such specifically named materials, since our enamel can be advantageously employed in conjunction with other mill additions and particularly other opacifiers. Thus, for example, we have been able to secure very satisfactory results indeed, by the use of so-called gas or bubble opacifiers and very good results have been secured by the combined useof gas opacifiers and zirconium opacifiers. As specific examples of gas opacifiers which have been found useful, we may mention aluminum stearate, aluminum acetate and ethylene glycol. Aluminum stearate will generally be found to produce best results when employed in concentrations of about 2-3 oz. per 100 lbs. of frit; ethylene glycol in the same concentrations, and aluminum acetate 4-5 oz. per 100 lbs. I

Other gas opacifiers which may be used to advantage are those given in U; S. Patent Nos. 1,948,461, 2,102,630, 2,103,228, 2,103,229, and 2,147,202.

Our improved enamel may be employed for application to either sheet metal or cast iron either wet or dry process) either a grip coat or a. conventional ground coat being usually employed for best results. The specific formulas given herein are primarily designed for use on sheet steel, since the compositions as given are inclined to be slightly too hard for use on cast iron. However, the composition may be readily modified to produce a softer firing enamel which may be more suitable for use on cast iron than those above given. As an indication of the very high degree of opacity obtainable by the use of our improved enamel, we might explain that single coat applications of 35-45 grams per square foot over a conventional sheet steel blue ground coat using approximately 3 lbs. per 100 lbs. of frit 'of zirconium opacifier as a mill addition and mate reflectance on the order of about 68 to It will thus beseen that the present enamel is admirably suited for the production of commercial articles by one cover coat of application.

While .it has been indicated above that there may be certain variations made in the composition of the raw mix from which our enamels are produced, it should, nevertheless, be pointed out that the zirconium and phosphorus content, as given in the several tables, has been found to be rather critical, both as to the amounts employed and the proportional relationship therebetween..

Other modes of applying the principle of our invention may be employed instead of the one explained, change being made as regards the method herein disclosed, provided the step or steps stated by any of the following claims or the equivalent of such stated step or steps be employed. A

We therefore particularly point out and distinctly claim as our invention:

1. A porcelain enamel frit of the character pro duced by smelting a raw batch mixture of the following composition:

2. A porcelain enamel frit of the character produced by smelting a raw batch mixture, the calculated oxide content of which comprises about:

MONROE J. BAHNSEN. EUGENE E. BRYANT. 

